The present invention relates to a polymeric material particularly adapted for use as a polymer electrolyte membrane in a fuel cell. Specifically, the present invention relates to rigid rod copolyimide polymers containing sulfonic acid groups. More specifically, the present invention relates to a copolyimide containing sulfonic acid constituents, having a rigid rod, liquid crystalline structure, and which incorporates bulky, displacing or angled monomers along the polymer chain. Incorporation of bulky monomers in the polymer chain creates regions of access along the chain, thereby exposing sulfonic acid groups, also located along the chain, which in turn enhances conductivity properties of the polymer.
Solid, proton conducting polymer electrolyte membranes (PEMs) provide several key features in present technology fuel cells. These features include providing a conduction medium for protons, supporting and separating electrodes, and separating fuel from oxidizer.
Consequently, polymer electrolyte membranes must exhibit ion exchange properties that allow sufficient conductivities to be achieved. In addition, such membranes must exhibit high chemical and mechanical resistance under extreme operating conditions which are typically encountered in many fuel cell applications.
An example of a commercially available polymer electrolyte membrane is Nafion(copyright) and is available from DuPont. Nafion(copyright) is adequate for use in most current fuel cell applications, but exhibits several deficiencies. It has poor conductivity at low relative humidities and can not easily be used at temperatures above 80xc2x0 C. because it dries out. Furthermore, Nafion(copyright) exhibits high osmotic drag which contributes to difficulties in water management at high current densities. In addition, high methanol permeability in Nafion(copyright) contributes to detrimental fuel cross over, in which fuel passes across the anode, through the Nafion(copyright) membrane and to the cathode. Consequently, in instances of fuel cross over, methanol is oxidized at the cathode and fuel cell efficiency decreases.
It would be beneficial to identify polymeric materials suitable for use as polymer electrolyte membranes in a fuel cell that exhibit improved properties over currently available materials. Specifically, it would be desirable to provide a polymeric material that exhibits increased conductivity, improved thermal stability, reduced methanol permeability and improved mechanical properties over presently available polymer electrolyte membranes such as Nafion(copyright).
The present invention achieves all of the foregoing objectives and provides, in a first aspect, a rigid rod ion conducting copolymer comprising three types of monomers. The first monomer is a generally non-polar monomer that is capable of binding a diamine. The second monomer is a diamine monomer having sulfonic acid groups. The third monomer is either a diamine or a dianhydride and is relatively bulky and capable of reacting with the first or second monomer. The copolymer exhibits liquid crystal behavior.
In another aspect, the present invention provides a polymer particularly adapted for use as a membrane in a fuel cell. The polymer exhibits a certain structure defined herein as either Structure I or II.
In a further aspect, the present invention provides a polymer electrolyte membrane comprising a polymer having either Structure I or II and a particular thickness range.
In an additional aspect, the present invention provides a fuel cell comprising a cathode, an anode, and a polymer electrolyte membrane situated between the cathode and anode. The polymer electrolyte membrane comprises a polymer having a structure defined herein as either Structure I or II.